This invention relates to optical fibers. More particularly, this invention relates to optical fibers in which both the core and sheath consist essentially of quartz glass.
Light conducting optical fibers are based on the principle that their internal reflectance approaches total internal reflectance when a core fiber has a transparent coating having a refractive index lower than that of the core and when the incident ray is within a particular acceptance angle .theta. , the maximum angle from the longitudinal axis at which an incident ray can be transmitted through the core fiber. The sine of this angle is defined as the numerical aperture (NA) given by the equation EQU sine .theta. = NA = .sqroot.N.sub.1.sup.2 - N.sub.2.sup.2
wherein N.sub.1 and N.sub.2 represent the core refractive index and the coating refractive index respectively.
Numerous methods are known in the art for producing optical fibers by enclosing a core of high refractive index within a sheath of lower refractive index. Core/sheath combinations which have been employed include synthetic plastic/synthetic plastic; multicomponent glass/multicomponent glass; multicomponent glass/synthetic plastic; quartz glass/synthetic plastic, etc. Additionally, optical fibers can be prepared by lowering the refractive index of the fiber sheath by ion-exchange and/or by giving the ion-exchanged fibers a refractive index profile, e.g., with a quadratic path.
Fiber optics composed of many optical fibers bundled together can take many forms, e.g., tortuous paths, faceplates, tapered fibers for magnification, etc. The bundles can be oriented to a coherent bundle capable of transmitting optical images or they can be unoriented light pipes capable of transmitting light, but not optical images. Quartz fibers have recently become available for transmission of ultraviolet light and are superior in many respects to glass or plastic fibers, but have heretofore required a glass or plastic sheath. The use of glass sheaths over a quartz glass core is disadvantageous, inter alia, in the different physical properties of the two materials, e.g., thermal expansion coefficients, which limit the areas of practical application. Plastic sheaths are susceptible to aging and oxidation, especially when exposed to ultraviolet light, and generally have poorer physical properties than quartz glass.
As is known, fiber dimensions are not critical provided their diameters are large in comparison to the wavelength of light to be transmitted and provided that the fibers in a bundle are spaced at least a wavelength apart to prevent light leakage from one to another.
Quartz glass can be produced with extremely low optical losses (5 - 10 dB/km at 850nm), while the lowest loss so far achieved with multi-component glass is about 50 dB/km at 850nm. The low losses of the quartz glass enable light transmission over large distances through a quartz glass optical fiber. Furthermore, data can be impressed on the transmitted light whereby the quartz glass optical fiber serves as a data transmission medium.
The production of a quartz glass optical fiber having a quartz glass sheath has not heretofore been possible because quartz glass has the lowest refractive index of all known glasses. Consequently, there is no available glass having a refractive index with which a quartz glass core could be covered to optically isolate the fiber. The possibility exists of enclosing the quartz glass fiber with a synthetic plastic sheath having a lower refractive index, e.g., Teflon. However, such fibers have transmission losses which lie in the same order of magnitude as the multi-component glass fibers. Thus, while they have the advantage of being transmissive for ultraviolet, fibers sheathed with synthetic plastics are unsuitable for transmitting light over large distances.
Attempts to increase the refractive index of quartz glass by doping it, inter alia, with TiO.sub.2, as described in German Offenlegungsschrift 2,122,896 have also been made. While an increase in the refractive index can be achieved by such doping, the transmission of the quartz glass is simultaneously deleteriously affected and this naturally is undesirable. A further particular disadvantage of the doping is that the ultraviolet transmission decreases; good ultraviolet transmission is typically the reason for which quartz glass is selected in the first place.